This invention is in the field of silicoaluminophosphate (SAPO) membranes, in particular SAPO membranes prepared on a porous support. The invention provides supported SAPO membranes as well as methods for making and using them.
SAPOs are largely composed of Si, Al, P and O and can have a three-dimensional microporous crystal framework structure of PO2+, AlO2− and SiO2 tetrahedral units. The cages, channels and cavities created by the crystal framework can permit separation of mixtures of molecules based on their effective sizes.
SAPO crystals can be synthesized by hydrothermal crystallization from a reaction mixture containing reactive sources of silica, alumina, and phosphate, and an organic templating agent. Lok et al. (U.S. Pat. No. 4,440,871) report gel compositions and procedures for forming several types of SAPO crystals, including SAPO-5, SAPO-11, SAPO-16, SAPO-17, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-37, SAPO-40, SAPO 41, SAPO-42, and SAPO-44 crystals. Lok et al. do not appear to disclose formation of SAPO membranes. Mériaudeu et al. (Mériaudeau, P. et al., J. Catalysis, 1997, 169, 55-66) report gel compositions and procedures for forming SAPO-11, SAPO-31, and SAPO-41 crystals. Mériaudeu et al. do not appear to disclose formation of SAPO membranes. Prakash and Unnikrishnan report gel compositions and procedures for forming SAPO-34 crystals. (Prakash, A. M. and Unnikrishnan, S., J. Chem. Sc. Faraday Trans., 1994, 90 (15), 2291-2296). In several of Prakash and Unnikrishnan's reported procedures, the gel was aged for 24 hours at 27° C. (300 K). Prakash and Unnikrishnan do not appear to disclose formation of SAPO-34 membranes.
SAPO membranes have been proposed for use in gas separations. For these applications, an important parameter is the separation selectivity. For two gas components i and j, a separation selectivity Si/j greater than one implies that the membrane is selectively permeable to component i. If a feedstream containing both components is applied to one side of the membrane, the permeate stream exiting the other side of the membrane will be enriched in component i and depleted in component j. The greater the separation selectivity, the greater the enrichment of the permeate stream in component i.
Barri et al. report supported zeolite membranes (U.S. Pat. No. 5,567,664) and methods for the production of zeolite membranes on porous supports (U.S. Pat. No. 5,362,522). Barri et al. state that any type of zeolite-type material may be used, including silicoaluminophosphates.
SAPO-5 and SAPO-11 membranes have been reported in the scientific literature. Sano et al. (Sano, T. et al., J. Mol. Cat. 1992, 77, L12) reported hydrothermal synthesis of SAPO-5 membranes on a Teflon slab. Sano et al. reported aging of the hydrogel overnight at room temperature before heating the substrate and gel. Tsai et al. (Tsai, T. G. et al., Micropor. Mesopor. Mat. 1998, 22, 333) reported synthesis of SAPO-5 membranes on anodic alumina supports using a microwave hydrothermal synthesis technique. Gump et al. (Gump, C. et al, Ind. Engr. Chem. Res, 2001, 40 (2), 565-577) reported hydrothermal synthesis of SAPO-5 and SAPO-11 membranes on the inner surface of α-alumina tubes with 200 nm pores.
SAPO-34 membranes on porous supports have been reported in the scientific literature. Lixiong et al. (Stud. Surf. Sci. Catl., 1997, 105, p 2211) reported synthesis of a SAPO-34 membrane on one side of a porous α-Al2O3 disk by immersing the substrate surface in a hydrogel and heating the substrate and gel. Lixiong et al. reported single gas permeances for H2, N2, CO2, and n-C4H10. Poshuta et al. (Ind. Eng. Chem. Res., 1998, 37, 3924-3929; AlChE Journal, 2000, 46 (4), 779-789) reported hydrothermal synthesis of SAPO-34 membranes on the inside surface of asymmetric, porous α-Al2O3 tubes. Poshuta et al. (supra) reported single gas and mixture permeances and ideal and mixture selectivities for several gases, including CO2 and CH4. The CO2/CH4 selectivities reported for a 50/50 CO2/CH4 mixture at 300 K were between 14 and 36 for a feed pressure of 270 kPa and a pressure drop of 138 kPa (Poshusta et al., AlChE Journal, 2000, 46 (4), pp 779-789). The CO2/CH4 selectivity was attributed to both competitive absorption (at lower temperatures) and differences in diffusivity.
There remains a need in the art for improved methods for making SAPO membranes, in particular SAPO membranes with improved separation selectivities.